Multi-port filter using transversely-excited film bulk acoustic resonators

ABSTRACT

Filter devices and methods are disclosed. A single-crystal piezoelectric plate is attached to substrate, portions of the piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate. A conductor pattern formed on the piezoelectric plate defines a low band filter including low band shunt resonators and low band series resonators and a high band filter including high band shunt resonators and high band series resonators. Interleaved fingers of interdigital transducers (IDTs) of the low band shunt resonators are disposed on respective diaphragms having a first thickness, interleaved fingers of IDTs of the high band series resonators are disposed on respective diaphragms having a second thickness less than the first thickness, and interleaved fingers of IDTs of the low band series resonators and the high band shunt resonators are disposed on respective diaphragms having thicknesses intermediate the first thickness and the second thickness.

RELATED APPLICATION INFORMATION

The patent claims priority to the following provisional patentapplications: application 63/040,435, titled MONOLITHIC MULTI-PORT XBAR,filed Jun. 17, 2020.

This patent is a continuation in part of application Ser. No.16/988,213, filed Aug. 7, 2020, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATORS WITH MULTIPLE DIAPHRAGM THICKNESSES AND FABRICATIONMETHOD, which claims priority to the following provisional patentapplications: application 62/892,980, titled XBAR FABRICATION, filedAug. 28, 2019; and application 62/904,152, titled DIELECTRIC OVERLAYERTRIMMING FOR FREQUENCY CONTROL, filed Sep. 23, 2019. Application Ser.No. 16/988,213 is a continuation in part of application Ser. No.16/438,121, filed Jun. 11, 2019, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, now U.S. Pat. No. 10,756,697, which is acontinuation-in-part of application Ser. No. 16/230,443, filed Dec. 21,2018, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, nowU.S. Pat. No. 10,491,192, which claims priority from the followingprovisional patent applications: application 62/685,825, filed Jun. 15,2018, entitled SHEAR-MODE FBAR (XBAR); application 62/701,363, filedJul. 20, 2018, entitled SHEAR-MODE FBAR (XBAR); application 62/741,702,filed Oct. 5, 2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR(XBAR); application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODEFILM BULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. All of these applications are incorporated herein byreference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1 .

FIG. 3 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1 .

FIG. 4 is a graphic illustrating a shear acoustic mode in an XBAR.

FIG. 5 is a schematic block diagram of a bandpass filter incorporatingseven XBARs.

FIG. 6A is a schematic cross-sectional view of a filter with adielectric layer to set a frequency separation between shunt resonatorsand series resonators.

FIG. 6B is a schematic cross-sectional view of a filter with differentpiezoelectric diaphragm thicknesses to set a frequency separationbetween shunt resonators and series resonators.

FIG. 7A is a schematic block diagram of a diplexer including twobandpass filters.

FIG. 7B is a schematic block diagram of a multiplexer including threebandpass filters.

FIG. 8 is a graph of the admittance of four XBARs for use in thediplexer of FIG. 7A.

FIG. 9 is a schematic cross-sectional view of a filter with threedifferent piezoelectric diaphragm thicknesses.

FIG. 10 is a more detailed schematic block diagram of a diplexerincluding two bandpass filters.

FIG. 11 is a more detailed schematic block diagram of a multiplexerincluding three bandpass filters.

FIG. 12 is a sequence of cross-sectional views illustrating a processfor forming a piezoelectric plate with multiple thicknesses.

FIG. 13 is a flow chart of a process for fabricating a filter.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, diplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the surfaces. However, XBARs may befabricated on piezoelectric plates with other crystallographicorientations.

The back surface 114 of the piezoelectric plate 110 is attached to asubstrate 120 that provides mechanical support to the piezoelectricplate 110. The substrate 120 may be, for example, silicon, sapphire,quartz, or some other material. The piezoelectric plate 110 may bebonded to the substrate 120 using a wafer bonding process, or grown onthe substrate 120, or attached to the substrate in some other manner.The piezoelectric plate may be attached directly to the substrate, ormay be attached to the substrate via one or more intermediate materiallayers.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites an acoustic wave within thepiezoelectric plate 110. As will be discussed in further detail, theexcited acoustic wave is a bulk shear wave that propagates in thedirection normal to the surface of the piezoelectric plate 110, which isalso normal, or transverse, to the direction of the electric fieldcreated by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

A cavity 140 is formed in the substrate 120 such that a portion 115 ofthe piezoelectric plate 110 containing the IDT 130 is suspended over thecavity 140 without contacting the substrate 120. “Cavity” has itsconventional meaning of “an empty space within a solid body.” The cavity140 may be a hole completely through the substrate 120 (as shown inSection A-A and Section B-B) or a recess in the substrate 120 (as shownsubsequently in FIG. 3 ). The cavity 140 may be formed, for example, byselective etching of the substrate 120 before or after the piezoelectricplate 110 and the substrate 120 are attached. As shown in FIG. 1 , thecavity 140 has a rectangular shape with an extent greater than theaperture AP and length L of the IDT 130. A cavity of an XBAR may have adifferent shape, such as a regular or irregular polygon. The cavity ofan XBAR may more or fewer than four sides, which may be straight orcurved.

The portion 115 of the piezoelectric plate suspended over the cavity 140will be referred to herein as the “diaphragm” (for lack of a betterterm) due to its physical resemblance to the diaphragm of a microphone.The diaphragm may be continuously and seamlessly connected to the restof the piezoelectric plate 110 around all, or nearly all, of perimeterof the cavity 140.

For ease of presentation in FIG. 1 , the geometric pitch and width ofthe IDT fingers is greatly exaggerated with respect to the length(dimension L) and aperture (dimension AP) of the XBAR. A typical XBARhas more than ten parallel fingers in the IDT 110. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 110.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100of FIG. 1 . The piezoelectric plate 110 is a single-crystal layer ofpiezoelectrical material having a thickness ts. ts may be, for example,100 nm to 1500 nm. When used in filters for LTE™ bands from 3.4 GHZ to 6GHz (e.g. bands 42, 43, 46), the thickness ts may be, for example, 200nm to 1000 nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2 , the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. tfd and tbd maybe, for example, 0 to 500 nm. tfd and tbd are typically less than thethickness is of the piezoelectric plate. tfd and tbd are not necessarilyequal, and the front-side and back-side dielectric layers 214, 216 arenot necessarily the same material. Either or both of the front-side andback-side dielectric layers 214, 216 may be formed of multiple layers oftwo or more materials.

The IDT fingers 238 may be aluminum or a substantially aluminum alloy,copper or a substantially copper alloy, beryllium, gold, or some otherconductive material. Thin (relative to the total thickness of theconductors) layers of other metals, such as chromium or titanium, may beformed under and/or over the fingers to improve adhesion between thefingers and the piezoelectric plate 110 and/or to passivate orencapsulate the fingers. The busbars (132, 134 in FIG. 1 ) of the IDTmay be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1 ) of the IDT may be the same as, or greater than, the thicknesstm of the IDT fingers.

FIG. 3 is an alternative cross-sectional view along the section planeA-A defined in FIG. 1 . In FIG. 3 , a piezoelectric plate 310 isattached to a substrate 320. An optional dielectric layer 322 may besandwiched between the piezoelectric plate 310 and the substrate 320. Acavity 340, which does not fully penetrate the substrate 320, is formedin the substrate under the portion of the piezoelectric plate 310containing the IDT of an XBAR. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings 342 provided in the piezoelectric plate310.

The XBAR 300 shown in FIG. 3 will be referred to herein as a “front-sideetch” configuration since the cavity 340 is etched from the front sideof the substrate 320 (before or after attaching the piezoelectric plate310). The XBAR 100 of FIG. 1 will be referred to herein as a “back-sideetch” configuration since the cavity 140 is etched from the back side ofthe substrate 120 after attaching the piezoelectric plate 110.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. An RF voltage is applied to the interleaved fingers 430. Thisvoltage creates a time-varying electric field between the fingers. Thedirection of the electric field is lateral, or parallel to the surfaceof the piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric field is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites a shear-mode acoustic mode, inthe piezoelectric plate 410. In this context, “shear deformation” isdefined as deformation in which parallel planes in a material remainparallel and maintain a constant distance while translating relative toeach other. A “shear acoustic mode” is defined as an acoustic vibrationmode in a medium that results in shear deformation of the medium. Theshear deformations in the XBAR 400 are represented by the curves 460,with the adjacent small arrows providing a schematic indication of thedirection and magnitude of atomic motion. The degree of atomic motion,as well as the thickness of the piezoelectric plate 410, have beengreatly exaggerated for ease of visualization. While the atomic motionsare predominantly lateral (i.e. horizontal as shown in FIG. 4 ), thedirection of acoustic energy flow of the excited primary shear acousticmode is substantially orthogonal to the surface of the piezoelectricplate, as indicated by the arrow 465.

Considering FIG. 4 , there is essentially no electric field immediatelyunder the IDT fingers 430, and thus acoustic modes are only minimallyexcited in the regions 470 under the fingers. There may be evanescentacoustic motions in these regions. Since acoustic vibrations are notexcited under the IDT fingers 430, the acoustic energy coupled to theIDT fingers 430 is low (for example compared to the fingers of an IDT ina SAW resonator), which minimizes viscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. Thus, high piezoelectric coupling enables thedesign and implementation of microwave and millimeter-wave filters withappreciable bandwidth.

FIG. 5 is a schematic circuit diagram for a high frequency band-passfilter 500 using XBARs. The filter 500 has a conventional ladder filterarchitecture including four series resonators 510A, 510B, 510C, 510D andthree shunt resonators 520A, 520B, 520C. The four series resonators510A, 510B, 510C, and 510D are connected in series between a first portand a second port. In FIG. 5 , the first and second ports are labeled“In” and “Out”, respectively. However, the filter 500 is symmetrical andeither port and serve as the input or output of the filter. The threeshunt resonators 520A, 520B, 520C are connected from nodes between theseries resonators to ground. All the shunt resonators and seriesresonators are XBARs. Although not shown in FIG. 5 , any and all of theresonators may be divided into multiple sub-resonators electricallyconnected in parallel. Each sub-resonator may have a respectivediaphragm.

The filter 500 may include a substrate having a surface, asingle-crystal piezoelectric plate having parallel front and backsurfaces, and an acoustic Bragg reflector sandwiched between the surfaceof the substrate and the back surface of the single-crystalpiezoelectric plate. The substrate, acoustic Bragg reflector, andpiezoelectric plate are represented by the rectangle 510 in FIG. 5 . Aconductor pattern formed on the front surface of the single-crystalpiezoelectric plate includes interdigital transducers (IDTs) for each ofthe four series resonators 510A, 510B, 510C, 510D and three shuntresonators 520A, 520B, 520C. All of the IDTs are configured to exciteshear acoustic waves in the single-crystal piezoelectric plate inresponse to respective radio frequency signals applied to each IDT.

In a ladder filter, such as the filter 500, the resonance frequencies ofshunt resonators are typically lower than the resonance frequencies ofseries resonators. The resonance frequency of an SM XBAR resonator isdetermined, in part, by IDT pitch. IDT pitch also impacts other filterparameters including impedance and power handling capability. Forbroad-band filter applications, it may not be practical to provide therequired difference between the resonance frequencies of shunt andseries resonators using only differences in IDT pitch.

As described in U.S. Pat. No. 10,601,392, a first dielectric layer(represented by the dashed rectangle 525) having a first thickness t1may be deposited over the IDTs of some or all of the shunt resonators520A, 520B, 520C. A second dielectric layer (represented by the dashedrectangle 515) having a second thickness t2, less than t1, may bedeposited over the IDTs of the series resonators 510A, 510B, 510C, 510D.The second dielectric layer may be deposited over both the shunt andseries resonators. The difference between the thickness t1 and thethickness t2 defines a frequency offset between the series and shuntresonators. Individual series or shunt resonators may be tuned todifferent frequencies by varying the pitch of the respective IDTs. Insome filters, more than two dielectric layers of different thicknessesmay be used as described in co-pending application Ser. No. 16/924,108.

Alternatively or additionally, the shunt resonators 510A, 510B, 510C,510D may be formed on a piezoelectric plate having a thickness t3 andthe series resonators may be fabricated on a piezoelectric plate havinga thickness t4 less than t3. The difference between the thicknesses t3and t4 defines a frequency offset between the series and shuntresonators. Individual series or shunt resonators may be tuned todifferent frequencies by varying the pitch of the respective IDTs. Insome filters, three or more different piezoelectric plate thicknessesmay be used to provide additional frequency tuning capability.

FIG. 6A is a schematic cross-sectional view though a shunt resonator anda series resonator of a filter 600A that uses dielectric thickness toseparate the frequencies of shunt and series resonators. A piezoelectricplate 610A is attached to a substrate 620. Portions of the piezoelectricplate form diaphragms spanning cavities 640 in the substrate 620.Interleaved IDT fingers, such as finger 630, are formed on thediaphragms. A first dielectric layer 650, having a thickness t1, isformed over the IDT of the shunt resonator. A second dielectric layer655, having a thickness t2, is deposited over both the shunt and seriesresonator. Alternatively, a single dielectric layer having thicknesst1+t2 may be deposited over both the shunt and series resonators. Thedielectric layer over the series resonator may then be thinned tothickness t2 using a masked dry etching process. In either case, thedifference between the overall thickness of the dielectric layers(t1+t2) over the shunt resonator and the thickness t2 of the seconddielectric layer defines a frequency offset between the series and shuntresonators.

The second dielectric layer 655 may also serve to seal and passivate thesurface of the filter 600A. The second dielectric layer may be the samematerial as the first dielectric layer or a different material. Thesecond dielectric layer may be a laminate of two or more sub-layers ofdifferent materials. Alternatively, an additional dielectric passivationlayer (not shown in FIG. 6A) may be formed over the surface of thefilter 600A. Further, as will be described subsequently, the thicknessof the final dielectric layer (i.e. either the second dielectric layer655 or an additional dielectric layer) may be locally adjusted tofine-tune the frequency of the filter 600A. Thus, the final dielectriclayer can be referred to as the “passivation and tuning layer”.

FIG. 6B is a schematic cross-sectional view though a shunt resonator anda series resonator of a filter 600B that uses piezoelectric platethickness to separate the frequencies of shunt and series resonators. Apiezoelectric plate 610B is attached to a substrate 620. Portions of thepiezoelectric plate form diaphragms spanning cavities 640 in thesubstrate 620. Interleaved IDT fingers, such as finger 630, are formedon the diaphragms. The diaphragm of the shunt resonator has a thicknesst3. The piezoelectric plate 610B is selectively thinned such that thediaphragm of the series resonator has a thickness t4, which is less thant3. The difference between t3 and t4 defines a frequency offset betweenthe series and shunt resonators. A passivation and tuning layer 655 isdeposited over both the shunt and series resonators.

FIG. 7A is a schematic block diagram of an exemplary multi-port filter.This example is a diplexer 700, or two-channel multiplexer, forseparating and/or combining radio frequency signals in two differentfrequency bands. A diplexer, such as the diplexer 700, may be used, forexample, to connect a low band transceiver and a high band transceiverto a common antenna as shown in FIG. 7 . The transceivers and antennaare not part of the diplexer 700.

The diplexer 700 includes a low band filter 710 and a high band filter720. The low band filter 710 has a passband that is lower in frequencyand does not overlap a passband of the high band filter 720. Both thelow band filter 710 and the high band filter 720 may be implementedusing XBARs connected in ladder filter circuits as shown in FIG. 5 .

FIG. 7B is a schematic block diagram of another exemplary multi-portfilter. This example is a multiplexer 750, for separating and/orcombining radio frequency signals in multiple different frequency bands.

The multiplexer 750 includes a low band filter 760, a high band filter770, and one or more mid band filter 780. In this context, “high” and“low” are relative terms. The high band filter 770 has the highestfrequency passband of any of the filters in the multiplexer. Similarly,the low band filter 760 has the lowest frequency passband of any of thefilters in the multiplexer. The one or more mid band filters 780 havepass band intermediate the passbands of the high and low band filters.The passbands of the filters do not overlap in frequency. The low bandfilter 760, the high band filter 770, and the one or more mid bandfilters 780 may be implemented using XBARs connected in ladder filtercircuits as shown in FIG. 5 .

FIG. 8 is a graph of the admittance of four representative XBARssuitable for use in a diplexer such as the diplexer 700 of FIG. 7 . FIG.8 assumes the low band and high band are nearly adjacent, with a smallfrequency difference between the upper band edge of the low band and thelow band edge of the high band. Diplexers with nearly adjacent bands maybe required when two transceivers are connected to a common antenna.Each of the four XBARs has a resonance frequency where its admittance ismaximum and an anti-resonance frequency where its admittance is minimum.

The solid curve 810 is a plot of the magnitude of admittance versusfrequency for a shunt resonator in the low band filter of the diplexer.The resonance frequency of this shunt resonator is just below the loweredge of the low band. The dashed curve 820 is a plot of the magnitude ofadmittance versus frequency for a series resonator in the low bandfilter of the diplexer. The resonance frequency of this series resonatoris just above the upper edge of the low band.

The dash-dot curve 830 is a plot of the magnitude of admittance versusfrequency for a shunt resonator in the high band filter of the diplexer.The resonance frequency of this shunt resonator is just below the loweredge of the high band. The solid curve 840 is a plot of the magnitude ofadmittance versus frequency for a series resonator in the high bandfilter of the diplexer. The resonance frequency of this series resonatoris just above the upper edge of the high band.

Depending on the frequencies of the low band and the high band, it maynot be possible to fabricate the shunt and series XBARs of both the highand low band filters on a single piezoelectric plate thickness. Forexample, consider a diplexer for 5G NR (5^(th) generation new radio)bands n77 (3300 MHz to 4200 MHz) and n79 (4400 MHz to 5000 MHz). Thedifference between the resonance frequency of the low band shuntresonator (curve 810) and the anti-resonance frequency of the high bandseries resonator (curve 840) is about 2 GHz. This frequency differencecannot be achieved on a single diaphragm thickness and is difficult toachieve with only two diaphragm thicknesses.

FIG. 9 is a schematic cross-sectional view of a portion of an XBARfilter 900 with three different piezoelectric diaphragm thicknesses. Thefilter 900 includes a piezoelectric plate 910 supported by a substrate920. Three portions of the piezoelectric plate 910 form diaphragms 915A,915B, 915C spanning cavities 940 is the substrate 920. Interleavedfingers of IDT (not identified) are formed on each diaphragm. Diaphragm915A has a first thickness ts1 equal to the original thickness of thepiezoelectric plate 910. Diaphragm 915A is the thickest of the threediaphragms. Diaphragm 915B has a second thickness ts2. Diaphragm 915B isthe thinnest of the three diaphragms. Diaphragm 915C has a thirdthickness ts3 which is intermediate ts1 and ts2. Diaphragms 915B and915C are areas where the piezoelectric plate was thinned by selectivelyremoving material from the surface of the plate. Processes for formingmultiple piezoelectric plate thicknesses which be describedsubsequently.

While FIG. 9 illustrates a filter using three diaphragm thicknesses,more than three diaphragm thicknesses are possible. By definition, thefirst diaphragm thickness ts1 is the thickest and the second diaphragmthickness is the thinnest. Other diaphragm thicknesses will beintermediate the first and second thickness, which is to say less thanthe first thickness and greater than the second thickness.

FIG. 10 is a schematic block diagram of a diplexer 1000, which may bethe diplexer 700 of FIG. 7A. Within the diplexer 1000, a low band filter1015 includes shunt resonators 1010A, 1010B and series resonators 1020A,1020B, 1020C connected in a ladder filter circuit. A high band filter1035 includes shunt resonators 1030A, 1030B and series resonators 1040A,1040B, 1040C connected in a ladder filter circuit. For example, the lowband filter 1015 may be a band n77 bandpass filter and the high bandfilter 1035 may be a band n79 bandpass filter.

The use of two shunt resonators and three series resonators for each oflow and high band filters is exemplary. Either filter may have more orfewer than two shunt resonators, more or fewer than three seriesresonators, and more or fewer than five total resonators. Port 1connects to the low band filter 1015, port 3 connects to the high bandfilter 1035, and port 2 is the common port.

The diplexer 1000 is implemented with XBARs with three or more differentdiaphragm thicknesses similar to the filter 900 shown in FIG. 9 . Thelow band shunt resonators (i.e. the shunt resonators of the low bandfilter 1015) 1010A and 1010B have a first diaphragm thickness ts1. Thehigh band series resonators have a second diaphragm thickness ts2, whichis less than ts1. The other resonators (low band series resonators andhigh band shunt resonators) have diaphragm thicknesses intermediate ts1and ts2. Referring back to FIG. 8 , the resonant frequencies of the lowband series resonators and the high band shunt resonators may berelatively close such that all of these resonators may have a commonthird diaphragm thickness ts3.

FIG. 11 is a schematic block diagram of a multiplexer 1100, which may bethe multiplexer 750 of FIG. 7B. Within the multiplexer 1100, a low bandfilter 1115 includes shunt resonators 1110A, 1110B and series resonators1120A, 1120B, 1120C connected in a ladder filter circuit. A high bandfilter 1135 includes shunt resonators 1130A, 1130B and series resonators1140A, 1140B, 1140C connected in a ladder filter circuit. A mid bandfilter 1155 includes shunt resonators 1150A, 1150B and series resonators1160A, 1160B, 1160C connected in a ladder filter circuit.

The use of two shunt resonators and three series resonators for each oflow, mid, and high band filters is exemplary. Each filter may have moreor fewer than two shunt resonators, more or fewer than three seriesresonators, and more or fewer than five total resonators. Port 1connects to the low band filter 1015, port 2 connects to the high bandfilter 1035, and port 3 connects to the mid band filter. All threefilters connect to the common port.

The multiplexer 1100 is implemented with XBARs with three or moredifferent diaphragm thicknesses similar to the filter 900 shown in FIG.9 . The low band shunt resonators (i.e. the shunt resonators of the lowband filter 1115) 1110A and 1110B have a first diaphragm thickness ts1.The high band series resonators 1140A, 1140B, 1140C have a seconddiaphragm thickness ts2, which is less than ts1. The other resonators(low band series resonators, all mid band resonators, and high bandshunt resonators) have diaphragm thicknesses intermediate ts1 and ts2.For example, the low band series 1120A, 1120B, 1120C resonators and themid band shunt resonators 1150A, 1150B may have diaphragm thickness ts3and the mid band series resonators 1160A, 1160B, 1160C and the high bandshunt resonators 1130A, 1130B may have diaphragm thickness ts4. In thiscase ts1>ts3>ts4>ts2.

Description of Methods

FIG. 12 is a series of schematic cross-section views illustrating aprocess 1200 to form different thickness areas of a piezoelectric plate.View A shows a piezoelectric plate 1210 with uniform thickness bonded toa substrate 1220. The piezoelectric plate 1210 may be, for example,lithium niobate or lithium tantalate. The substrate 1220 may be asilicon wafer or some other material as previously described.

View B illustrates selective removal to thin selected portions of thepiezoelectric plate. Selected portions of the piezoelectric plate may bethinned, for example, to provide diaphragms for series resonators aspreviously shown in FIG. 6B. Selected portions of the piezoelectricplate may be thinned using a scanning ion mill or other scanningmaterial removal tool if the tool has sufficient spatial resolution todistinguish the areas of the piezoelectric plate to be thinned.Alternatively, a scanning or non-scanning material removal tool 1250 maybe used to remove material from portions of the surface of thepiezoelectric plate defined by a mask 1252. The material removal toolmay be, for example, a tool employing Fluorine-based reactive ionetching, a sputter etching tool, or some other dry etching tool.Alternatively, a wet etch may be used to selectively remove materialfrom areas defined by the mask 1252. When more than two differentdiaphragm thickness are required, multiple material operations withdifferent masks may be performed.

View C illustrates a piezoelectric plate 1210 with three differentthicknesses bonded to a substrate 1220. The piezoelectric plate 1210 hasan original surface 1260 and two recesses 1262, 1264 with differentthicknesses formed by removing material as shown in view B. The process1200 may be used to form areas on the piezoelectric plate with more thanthree different thicknesses. Cavities may then be formed under portionsof these areas to provide diaphragms with three different thicknesses.The process 1200 is exemplary and other different processes and methodsmay be used to create multiple thickness regions on a piezoelectricplate. The process 1200 is not limited to three different thicknessesand may be used to create piezoelectric plates with areas having four ormore different thicknesses.

FIG. 13 is a simplified flow chart showing a process 1300 forfabricating a filter device incorporating XBARs. Specifically, theprocess 1300 is for fabricating a filter device using a frequencysetting dielectric layer over some resonators as shown in FIG. 6A andmultiple diaphragm thicknesses as shown in FIG. 6B and FIG. 9 . Theprocess 1300 starts at 1305 with a device substrate and a thin plate ofpiezoelectric material disposed on a sacrificial substrate. The process1300 ends at 1395 with a completed filter device. The flow chart of FIG.13 includes only major process steps. Various conventional process steps(e.g. surface preparation, cleaning, inspection, baking, annealing,monitoring, testing, etc.) may be performed before, between, after, andduring the steps shown in FIG. 13 .

While FIG. 13 generally describes a process for fabricating a singlefilter device, multiple filter devices may be fabricated simultaneouslyon a common wafer (consisting of a piezoelectric plate bonded to asubstrate). In this case, each step of the process 1300 may be performedconcurrently on all of the filter devices on the wafer.

The flow chart of FIG. 13 captures three variations of the process 1300for making an XBAR which differ in when and how cavities are formed inthe device substrate. The cavities may be formed at steps 1310A, 1310B,or 1310C. Only one of these steps is performed in each of the threevariations of the process 1300.

The piezoelectric plate may be, for example, lithium niobate or lithiumtantalate, either of which may be Z-cut, rotated Z-cut, or rotatedYX-cut. The piezoelectric plate may be some other material and/or someother cut. The device substrate may preferably be silicon. The devicesubstrate may be some other material that allows formation of deepcavities by etching or other processing.

In one variation of the process 1300, one or more cavities are formed inthe device substrate at 1310A, before the piezoelectric plate is bondedto the substrate at 1315. A separate cavity may be formed for eachresonator in a filter device. The one or more cavities may be formedusing conventional photolithographic and etching techniques. Typically,the cavities formed at 1310A will not penetrate through the devicesubstrate, and the resulting resonator devices will have a cross-sectionas shown in FIG. 3 .

At 1315, the piezoelectric plate is bonded to the device substrate. Thepiezoelectric plate and the device substrate may be bonded by a waferbonding process. Typically, the mating surfaces of the device substrateand the piezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the device substrate. One or both mating surfaces may beactivated using, for example, a plasma process. The mating surfaces maythen be pressed together with considerable force to establish molecularbonds between the piezoelectric plate and the device substrate orintermediate material layers.

At 1320, the sacrificial substrate may be removed. For example, thepiezoelectric plate and the sacrificial substrate may be a wafer ofpiezoelectric material that has been ion implanted to create defects inthe crystal structure along a plane that defines a boundary between whatwill become the piezoelectric plate and the sacrificial substrate. At1320, the wafer may be split along the defect plane, for example bythermal shock, detaching the sacrificial substrate and leaving thepiezoelectric plate bonded to the device substrate. The exposed surfaceof the piezoelectric plate may be polished or processed in some mannerafter the sacrificial substrate is detached.

Thin plates of single-crystal piezoelectric materials laminated to anon-piezoelectric substrate are commercially available. At the time ofthis application, both lithium niobate and lithium tantalate plates areavailable bonded to various substrates including silicon, quartz, andfused silica. Thin plates of other piezoelectric materials may beavailable now or in the future. The thickness of the piezoelectric platemay be between 300 nm and 1000 nm. When the substrate is silicon, alayer of SiO₂ may be disposed between the piezoelectric plate and thesubstrate. When a commercially available piezoelectric plate/devicesubstrate laminate is used, steps 1310A, 1315, and 1320 of the process1300 are not performed.

At 1330, selected areas of the piezoelectric plate are thinned. Forexample, areas of the piezoelectric plate that will become thediaphragms of series resonators may be thinned as shown in FIG. 12 . Thethinning may be done using a scanning material tool such as an ion mill.Alternatively, the areas to be thinned may be defined by a mask andmaterial may be removed using an ion mill, a sputter etching tool, or awet or dry etching process. In all cases, precise control of the depthof the material removed over the surface of a wafer is required. Afterthinning, the piezoelectric plate will be divided into regions havingtwo or more different thicknesses.

The surface remaining after material is removed from the piezoelectricplate may be damaged, particularly if an ion mill or sputter etch toolis used at 1330. Some form of post processing, such as annealing orother thermal process may be performed at 1335 to repair the damagedsurface.

After the piezoelectric plate is selectively thinned at 1330 and anysurface damage is repaired at 1335, a first conductor pattern, includingIDTs of each XBAR, is formed at 1345 by depositing and patterning one ormore conductor layers on the front side of the piezoelectric plate. Theconductor layer may be, for example, aluminum, an aluminum alloy,copper, a copper alloy, or some other conductive metal. Optionally, oneor more layers of other materials may be disposed below (i.e. betweenthe conductor layer and the piezoelectric plate) and/or on top of theconductor layer. For example, a thin film of titanium, chrome, or othermetal may be used to improve the adhesion between the conductor layerand the piezoelectric plate. A second conductor pattern of gold,aluminum, copper or other higher conductivity metal may be formed overportions of the first conductor pattern (for example the IDT bus barsand interconnections between the IDTs).

Each conductor pattern may be formed at 1345 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, or other etching techniques.

Alternatively, each conductor pattern may be formed at 1345 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 1350, one or more frequency setting dielectric layer(s) may be formedby depositing one or more layers of dielectric material on the frontside of the piezoelectric plate. The one or more dielectric layers maybe deposited using a conventional deposition technique such as physicalvapor deposition, atomic layer deposition, chemical vapor deposition, orsome other method. One or more lithography processes (using photomasks)may be used to limit the deposition of the dielectric layers to selectedareas of the piezoelectric plate. For example, a mask may be used tolimit a dielectric layer to cover only selected resonators.

At 1355, a passivation/tuning dielectric layer is deposited over thepiezoelectric plate and conductor patterns. The passivation/tuningdielectric layer may cover the entire surface of the filter except forpads for electrical connections to circuitry external to the filter. Insome instantiations of the process 1300, the passivation/tuningdielectric layer may be formed after the cavities in the devicesubstrate are etched at either 1310B or 1310C.

In a second variation of the process 1300, one or more cavities areformed in the back side of the device substrate at 1310B. A separatecavity may be formed for each resonator in a filter device. The one ormore cavities may be formed using an anisotropic ororientation-dependent dry or wet etch to open holes through the backside of the device substrate to the piezoelectric plate. In this case,the resulting resonator devices will have a cross-section as shown inFIG. 1 .

In a third variation of the process 1300, one or more cavities in theform of recesses in the device substrate may be formed at 1310C byetching the substrate using an etchant introduced through openings inthe piezoelectric plate. A separate cavity may be formed for eachresonator in a filter device. The one or more cavities formed at 1310Cwill not penetrate through the device substrate, and the resultingresonator devices will have a cross-section as shown in FIG. 3 .

Ideally, after the cavities are formed at 1310B or 1310C, most or all ofthe filter devices on a wafer will meet a set of performancerequirements. However, normal process tolerances will result invariations in parameters such as the thicknesses of dielectric layerformed at 1350 and 1355, variations in the thickness and line widths ofconductors and IDT fingers formed at 1345, and variations in thethickness of the PZT plate. These variations contribute to deviations ofthe filter device performance from the set of performance requirements.

To improve the yield of filter devices meeting the performancerequirements, frequency tuning may be performed by selectively adjustingthe thickness of the passivation/tuning layer deposited over theresonators at 1355. The frequency of a filter device passband can belowered by adding material to the passivation/tuning layer, and thefrequency of the filter device passband can be increased by removingmaterial to the passivation/tuning layer. Typically, the process 1300 isbiased to produce filter devices with passbands that are initially lowerthan a required frequency range but can be tuned to the desiredfrequency range by removing material from the surface of thepassivation/tuning layer.

At 1360, a probe card or other means may be used to make electricalconnections with the filter to allow radio frequency (RF) tests andmeasurements of filter characteristics such as input-output transferfunction. Typically, RF measurements are made on all, or a largeportion, of the filter devices fabricated simultaneously on a commonpiezoelectric plate and substrate.

At 1365, global frequency tuning may be performed by removing materialfrom the surface of the passivation/tuning layer using a selectivematerial removal tool such as, for example, a scanning ion mill aspreviously described. “Global” tuning is performed with a spatialresolution equal to or larger than an individual filter device. Theobjective of global tuning is to move the passband of each filter devicetowards a desired frequency range. The test results from 1360 may beprocessed to generate a global contour map indicating the amount ofmaterial to be removed as a function of two-dimensional position on thewafer. The material is then removed in accordance with the contour mapusing the selective material removal tool.

At 1370, local frequency tuning may be performed in addition to, orinstead of, the global frequency tuning performed at 1365. “Local”frequency tuning is performed with a spatial resolution smaller than anindividual filter device. The test results from 1360 may be processed togenerate a map indicating the amount of material to be removed at eachfilter device. Local frequency tuning may require the use of a mask torestrict the size of the areas from which material is removed. Forexample, a first mask may be used to restrict tuning to only shuntresonators, and a second mask may be subsequently used to restricttuning to only series resonators (or vice versa). This would allowindependent tuning of the lower band edge (by tuning shunt resonators)and upper band edge (by tuning series resonators) of the filter devices.

After frequency tuning at 1365 and/or 1370, the filter device iscompleted at 1375. Actions that may occur at 1375 include formingbonding pads or solder bumps or other means for making connectionbetween the device and external circuitry (if such pads were not formedat 1345); excising individual filter devices from a wafer containingmultiple filter devices; other packaging steps; and additional testing.After each filter device is completed, the process ends at 1395.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A filter device, comprising: a substrate having asurface; a single-crystal piezoelectric plate having front and backsurfaces, the back surface attached to the surface of the substrate,portions of the single-crystal piezoelectric plate forming a pluralityof diaphragms spanning respective cavities in the substrate; and aconductor pattern formed on the front surface, the conductor patterndefining a low band filter including one or more low band shuntresonators and one or more low band series resonators and a high bandfilter including one or more high band shunt resonators and one or morehigh band series resonators, wherein interleaved fingers of interdigitaltransducers (IDTs) of the one or more low band shunt resonators aredisposed on respective diaphragms having a first thickness ts1,interleaved fingers of IDTs of the one or more high band seriesresonators are disposed on respective diaphragms having a secondthickness ts2 less than ts1, and interleaved fingers of IDTs of the oneor more low band series resonators and the one or more high band shuntresonators are disposed on respective diaphragms having otherthicknesses intermediate ts1 and ts2.
 2. The filter device of claim 1,wherein interleaved fingers of IDTs of the one or more low band seriesresonators and the one or more high band shunt resonators are disposedon respective diaphragms having a third thickness ts3 intermediate ts1and ts2.
 3. The filter device of claim 1, wherein the single-crystalpiezoelectric plate and all of the IDTs are configured such that arespective radio frequency signal applied to each IDT excites arespective shear primary acoustic mode within the respective diaphragm.4. The filter device of claim 3, wherein a direction of acoustic energyflow of all of the shear primary acoustic modes is substantiallyorthogonal to the front and back surfaces of the respective diaphragms.5. The filter device of claim 3, wherein the single-crystalpiezoelectric plate is one of lithium niobate and lithium tantalate. 6.The filter device of claim 1, the conductor pattern further defining oneor more mid band filters including one or more mid band shunt resonatorsand one or more mid band series resonators, wherein interleaved fingersof IDTs of the one or more mid band series resonators and the one ormore mid band shunt resonators are disposed on respective diaphragmshaving other thicknesses intermediate ts1 and ts2.
 7. The filter deviceof claim 1, the conductor pattern further defining a mid band filterincluding one or more mid band shunt resonators and one or more mid bandseries resonators, wherein interleaved fingers of IDTs of the one ormore low band series resonators and the one or more mid band shuntresonators are disposed on respective diaphragms having a thirdthickness ts3, interleaved fingers of the IDTs of the one or more midband series resonators and the one or more high band shunt resonatorsare disposed on respective diaphragms having a fourth thickness ts4, andts1>ts3>ts4>ts2.
 8. The filter device of claim 1, wherein the low bandfilter is a band n77 bandpass filter, and the high band filter is a bandn79 bandpass filter.
 9. A method of fabricating a filter device,comprising: attaching a back surface of a piezoelectric plate havingopposing front and back surfaces and a first thickness ts1 to a surfaceof a substrate; selectively thinning portions of the piezoelectric platefrom the first thickness ts1 to a second thickness ts2 less than thefirst thickness; selectively thinning additional portions of thepiezoelectric plate from the first thickness to one or more additionalthicknesses intermediate ts1 and ts2; forming cavities in the substratesuch that portions of the single-crystal piezoelectric plate form aplurality of diaphragms spanning respective cavities; and forming aconductor pattern on the front surface, the conductor pattern defining alow band filter including one or more low band shunt resonators and oneor more low band series resonators and a high band filter including oneor more high band shunt resonators and one or more high band seriesresonators, wherein interleaved fingers of interdigital transducers(IDTs) of the one or more low band shunt resonators are disposed onrespective diaphragms having the first thickness ts1, interleavedfingers of IDTs of the one or more high band series resonators aredisposed on respective diaphragms having the second thickness ts2, andinterleaved fingers of IDTs of the one or more low band seriesresonators and the one or more high band shunt resonators are disposedon respective diaphragms having one of the one or more additionalthicknesses intermediate ts1 and ts2.
 10. The method of claim 9, whereinthe one or more additional thicknesses consists of a third thicknessintermediate ts1 and ts2, and interleaved fingers of IDTs of the one ormore low band series resonators and the one or more high band shuntresonators are disposed on respective diaphragms having the thirdthickness.
 11. The method of claim 9, wherein the single-crystalpiezoelectric plate and all of the IDTs are configured such that arespective radio frequency signal applied to each IDT excites arespective shear primary acoustic mode within the respective diaphragm.12. The method of claim 11, wherein a direction of acoustic energy flowof all of the shear primary acoustic modes is substantially orthogonalto the front and back surfaces of the respective diaphragms.
 13. Themethod of claim 11, wherein the single-crystal piezoelectric plate isone of lithium niobate and lithium tantalate.
 14. The method of claim 9,the conductor pattern further defining one or more mid band filtersincluding one or more mid band shunt resonators and one or more mid bandseries resonators, wherein interleaved fingers of IDTs of the one ormore mid band series resonators and the one or more mid band shuntresonators are disposed on respective diaphragms having otherthicknesses intermediate ts1 and ts2.
 15. The method of claim 9, theconductor pattern further defining a mid band filter including one ormore mid band shunt resonators and one or more mid band seriesresonators, wherein interleaved fingers of IDTs of the one or more lowband series resonators and the one or more mid band shunt resonators aredisposed on respective diaphragms having a third thickness ts3,interleaved fingers of the IDTs of the one or more mid band seriesresonators and the one or more high band shunt resonators are disposedon respective diaphragms having a fourth thickness ts4, andts1>ts3>ts4>ts2.
 16. The method of claim 9, wherein the low band filteris a band n77 bandpass filter, and the high band filter is a band n79bandpass filter.